The invention concerns a method for automatically controlling a stationary gas engine, in which a speed control deviation is computed from a set speed and an actual speed, the speed control deviation is used by a speed controller to determine a set torque as a correcting variable, and a torque limiter limits the set torque to an air ratio limit torque. In addition, the method consists in determining a set volume flow from the limited set torque in order to determine a mixture throttle angle and a gas throttle angle.
Stationary gas engines are used for electric power generation. In this connection, the gas engine is operated at a lambda value of, for example, 1.7, i.e., a lean mixture with excess air. The gas engine typically includes a gas throttle for setting the gas fraction in the gas/air mixture, a mixer for mixing the combustible gas and the air, a compressor as part of an exhaust gas turbocharger, a cooler, and a mixture throttle. The intake volume flow in the receiver tube upstream of the intake valves of the gas engine is set by the mixture throttle, and thus the mixture pressure in the receiver tube is also set.
EP 1 158 149 A1 describes a stationary gas engine for driving a generator. The gas engine is controlled by using a characteristic curve to compute a set lambda as a reference input from the engine output. On the basis of the set lambda, an electronic engine control unit computes a gas quantity set value for adjusting the gas throttle. In a second embodiment, the set lambda value is computed from a mixture pressure control deviation. The mixture pressure control deviation is determined from the detected actual mixture pressure in the receiver tube and the set mixture pressure, which in turn is determined from the engine output by means of a characteristic curve. In a third embodiment, as a supplement to the second embodiment, the gas quantity set value is corrected to adjust the gas throttle as a function of the position of a compressor bypass valve and the speed control deviation. A common feature of all three embodiments is the adjustment of the gas throttle to a set lambda value. In practical operation, this means that when a change in the power assignment is made, first a change is made in the position of the mixture throttle as the power control element. This has the effect that the intake mixture volume flow also changes. Since the position of the gas throttle initially remains constant, there is also no change in the gas volume flow. This results in a changing actual lambda. When a mixture throttle is controlled to move, for example, in the closing direction, this causes enrichment of the mixture, which results in a change in output of the gas engine. As a response to this change in output, the set lambda value, the gas quantity set value, and the position of the gas throttle are then changed. In this type of automatic control, the response time, for example, when the load changes, is critical, since intervention in the lambda control is sluggish due to the system itself.
DE 103 46 983 A1 also describes a gas engine and a method for automatically controlling the fuel mixture. In this method, in a first step, an actual pressure difference of the air mass flow is determined in a venturi mixer, and, in a second step, a set pressure difference of the air mass flow is determined from the measured actual output of the gas engine. In a third step, the actual pressure difference is then brought closer to the set pressure difference by changing the amount of gas supplied by changing the position of the gas throttle. In a fourth step, the actual gas engine output that develops is detected again, and the mixture throttle is adjusted in such a way that the set/actual deviation of the pressure difference of the air mass flow in the venturi mixer is reduced. This sequential order of operations is carried out iteratively until the set/actual deviation of the pressure difference is smaller than a limit. Since a change in the position of the mixture throttle produces a change in the output of the gas engine, the position of the gas throttle must be readjusted to compensate the change in output of the gas engine. Under certain circumstances, this can cause the correcting variables to overshoot.
A method for automatically controlling a stationary gas engine is disclosed by the nonprepublished German patent application with the official file number DE 10 2007 045 195.9, in which a gas throttle and a mixture throttle are simultaneously controlled as a function of the same actuating variable, in this case a set volume flow. The set volume flow is determined on the basis of a set torque, which is computed as a correcting variable by a speed controller from a set/actual deviation of the engine speeds. To improve the operating reliability, a torque limiter is provided, by which the set torque is limited as a function of a fault signal, a permissible mechanical maximum torque, and the actual engine speed. The fault signal is set, for example, in the event of a sensor failure. Due to the high dynamics during a load change, this automatic control system can lead to a mixture than is too rich. A mixture that is too rich causes excessive component stress, an elevated combustion temperature, and thus worse exhaust gas values.